Enable load balancing policy extensions (#17400)
Enables `LOAD_BALANCING_POLICY_CONFIG` enum value in `LbPolicy` and supports typed load balancers specified in `load_balancing_policy`. Continues work done by Charlie Getzen <charliegetzenlc@gmail.com> in #15827. Custom load balancers specified by `load_balancing_policy` are created as implementations of `ThreadAwareLoadBalancer`. Thread-local load balancers can be implemented as thread-aware load balancers that contain no logic at the thread-aware level, i.e. the purpose of the thread-aware LB is solely to contain the factory used to instantiate the thread-local LBs. (In the future it might be appropriate to provide a construct that abstracts away thread-aware aspects of `ThreadAwareLoadBalancer` for LBs that don't need to be thread-aware.) A cluster that uses `LOAD_BALANCING_POLICY_CONFIG` may not also set a subset LB configuration. If the load balancer type makes use of subsetting, it should include a subset configuration in its own configuration message. Future work on load balancing extensions should include moving the subset LB to use load balancing extensions. Similarly, a cluster that uses `LOAD_BALANCING_POLICY_CONFIG` may not set the `CommonLbConfig`, and it is not passed into load balancer creation (mostly owing to its dubious applicability as a top level configuration message to hierarchical load balancing policy). If the load balancer type makes use of the `CommonLbConfig`, it should include a `CommonLbConfig` in the configuration message for the load balancing policy. Considerations for migration of existing load balancers: - pieces of the `ThreadAwareLoadBalancerBase` implementation are specific to the built-in hashing load balancers and should be moved into a base class specifically for hashing load balancers. As it stands, custom load balancing policies are required to implement a `createLoadBalancer()` method even if the architecture of the LB policy does not require a hashing load balancer. I think we would also benefit from disentangling `ThreadAwareLoadBalancerBase` from `LoadBalancerBase`, as the former never actually does any host picking. - as we convert existing thread-local load balancers to thread-aware load balancers, new local LBs will be re-created upon membership changes. We should provide a mechanism allowing load balancers to control whether this rebuild should occur, e.g. a callback that calls `create()` for thread-aware LBs by default, which can be overridden to do nothing for thread-local LBs. Risk Level: low Testing: brought up a cluster with a custom load balancer specified by `load_balancing_policy`; new unit tests included Docs Changes: n/a Release Notes: Enable load balancing policy extensions Platform Specific Features: n/a Fixes #5598 Signed-off-by: Eugene Chan <eugenechan@google.com> Mirrored from https://github.com/envoyproxy/envoy @ e31bc1edec3bb80760a1442869cc442d123e9adcpull/624/head
data-plane-api(Azure Pipelines)
4 years ago
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ccd5246d84
2 changed files with 14 additions and 22 deletions
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